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SOLUTIONS TO EXERCISE 18
SOLUTIONS TO EXERCISE 18 Since Venus, Earth and the Sun move relative to each other, Venus can also have phases as seen from Earth. Refer to diagrams at the URL http://astrosun.tn.cornell.edu/courses/astro201/venus_phase.htm for details. One might expect that, since Venus takes 243 days to go around the Sun once, that a quarter phase of Venus would take one quarter of that time, that is about 61 days. However, since Earth takes 365 days to go around the Sun once, and it's moving in the same direction as Venus, both the orbits of Earth and Venus must be taken into account to compute the duration of phases. The "synodic period" is the amount of time a full cycle of phases will occur; this is about 584 days. (See, for example, the URLs http://www.mira.org/fts0/planets/093/text/txt001x.htm or http://www.ssec.wisc.edu/outreach/SPARK/venus.html for more details.) So the time we see Venus go from new to first quarter is 584/4 = 146 days. |